The present invention relates to a frame memory for use in a motion picture decoder, and more particularly, to a frame memory which can perform motion compensation at a high speed and in which a synchronous dynamic random access memory (SDRAM) is used to store a reference picture.
Generally, a system such as a high-definition TV or a digital VCR encodes and decodes digital audio and video signals. A video encoder performs orthogonal transformation, quantization, variable-length coding, and motion estimation and compensation coding, on input video signals.
FIG. 1 shows an apparatus for decoding video data encoded by the above video encoder. In the FIG. 1 video decoder, a variable-length decoder 11 variable-length-decodes received encoded data. An inverse quantizer 12 inversely quantizes the variable-length-decoded data. An inverse discrete cosine transformer (inverse DCT) 13 transforms the inversely quantized data into video data having a spatial domain. A motion compensator 14 reads video data of a macroblock corresponding to a motion vector from a frame memory 15 and performs motion compensation with respect to the video data supplied from the inverse DCT 13. The motion compensated video data is output to a downstream block (not shown) and also stored in the frame memory 15 which will be used for subsequent motion compensation. Here, the motion vector is output from the variable-length decoder 11 and is usually supplied from an encoder, together with the encoded video data.
Prediction comprises reading out data of a predictive macroblock designated by a motion vector from the frame memory 15. There are two kinds of prediction. One is a "field prediction" with respect to a reference field picture stored in the frame memory 15, and the other is a "frame prediction" with respect to a reference frame picture stored in the frame memory 15. A picture is classified into a "field picture" which is decoded or encoded in units of a field, and a "frame picture" which is decoded or encoded in units of a frame. Two field pictures, including a top and bottom field, correspond to one frame. The field picture is used only for field prediction and the frame picture is used for both field prediction and frame prediction. To process such prediction without delay, data stored in the frame memory 15, should be swiftly read out. However, as the motion between pictures increases, a larger amount of data should be read out from the frame memory 15. Therefore, it is necessary that a frame memory 15 should swiftly output stored data.